Plasma processing techniques have found wide-spread use in industry for commercial processes such as plasma vapor deposition, sputtering, and so forth. These processes have become particularly useful in thin film applications. To generate plasma, a power supply creates an electric potential between a cathode and one or more anodes that are placed in a plasma chamber containing the process gases that form the plasma. When using these processes for deposition, the plasma acts upon the material of a target (also referred to as a sputtering source) placed in the plasma chamber that normally comprises the cathode surface. Plasma ions are accelerated towards the target and cause target material to be dislodged from the cathode surface on impact. The dislodged target material is then deposited on a substrate to form a film (e.g., thin film). The film may constitute material sputtered by the plasma from the target surface, as disclosed above, or may be the result of a reaction between the target material and some other element included in the plasma or process gases.
High frequency voltage sources (e.g., alternating-current (AC) power sources) have been used to generate a high electrical potential that produces a plasma within a plasma chamber. These high-frequency voltage sources are expensive to construct and maintain, as well as complicated to operate. Additionally, if the deposition material is formed by reaction with an element in the plasma or process gases, and further, is electronically insulating, the anode in the chamber can be coated with the insulator; this deposit can then prevent the anode from performing its function of collecting electrons from the plasma during the deposition process.
To overcome the disadvantages associated with high frequency voltage sources, alternating pulsed direct current power sources have been employed in anodeless dual magnetron-type systems such as that disclosed in U.S. Pat. No. 5,917,286, which is incorporated herein by reference in its entirety. The process of reversing polarities allows the electrodes to alternately act as an anode and as a cathode, and the sputtering process that occurs during the cathode phase cleans off any deposited insulating material and permits uninhibited operation of the electrode as an anode during the anode phase. Additionally, the process of reversing polarities allows both electrodes to alternatively act as a cathode so that both electrode surfaces are capable of providing target material.
Although the present pulsed direct current power sources are functional, they are not sufficiently accurate or otherwise satisfactory in many thin film processing applications to achieve, for example, uniformity and/or particle generation thresholds. Co-sputtering of an arbitrary stoichiometry is also not achievable with standard technology. Accordingly, methods and apparatus are needed to address the shortfalls of present technology and to provide other new and innovative features.